1. Technical Field
This invention relates to the technology of engine exhaust gas purification, and more particularly to the technology of removing NO.sub.x at high efficiency in an oxygen-rich atmosphere from lean side air/fuel (A/F) ratio combustion.
2. Discussion of the Prior Art
Much of the prior art catalysts were designed to operate in a somewhat oxygen-deficient environment since engines were operated at or about stoichiometric combustion. Optimally, commercially available automotive catalysts will not promote reduction of NO.sub.x in an environment of excess oxygen; the-traditional reductant, rhodium, is restricted to a narrow window of A/F if ammonia production is to be avoided.
High silica zeolites (2XM/.sub.n.XAl.sub.2 O.sub.3.YSiO.sub.2) of the transition metal ion exchange type, when loaded on an alumina carrier, have demonstrated a desirable ability to act as a molecular sieve and create active sites (a surface on which NO can compete with O.sub.2 for reacting with a reductant), thus permitting reduction of NO.sub.x to take place in an oxidizing environment.
One of the earliest applications of high silica zeolites to the purification of engine exhaust gases is disclosed in U.S. Pat. No. 4,297,328, wherein a copper exchanged zeolite is deployed. The copper is most effective as the ion exchange metal because it is active at lower temperatures (such as present in a lean-burn engine exhaust) than other metals known to date. Such catalyst was used to perform as a three-way catalyst in an oxidizing environment and was found to initially possess a high absorption capacity for organic materials at high temperatures without preference for water.
Unfortunately, transition metal containing zeolites degrade at high temperatures usually found in automotive exhaust systems. The catalyst, that is, the transition metal, tends to react with the alumina of the zeolite and form an aluminate which acts as a low surface area material preventing the transition metal from being actively catalytic and thereby reduces the catalytic activity of the entire system.